JVMTop

http://code.google.com/p/jvmtop

It's not a spectacular tool, but I think it can be added to the toolbox of an Operations guy.

It quickly gives you an overview of all the running JVMs, with number of threads, GC overhead and CPU utilized.

I like the option of single PID mode (./jvmtop.sh PID), since it shows you the top 10 threads eating more CPU. Quite useful for a quick debugging.